Spatial profiles and scale lengths of nickel, iron, and CN on the interstellar comet 3I/ATLAS

¹ Università degli Studi di Padova (UNIPD), Dipartimento di Fisica e Astronomia “G. Galilei”, Via F. Marzolo 8, 35131 Padova, Italy
² Istituto Nazionale di Astrofisica (INAF) – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
³ Koyama Space Science Institute, Kyoto Sangyo University, Kamigamo Motoyama, Kita-ku, Kyoto, Kyoto 603-8555, Japan
⁴ Division of Science, Kyoto Sangyo University, Kamigamo Motoyama, Kita-ku, Kyoto, Kyoto 603-8555, Japan
⁵ Photocross Co., Ltd., Bldg. D, Iwakura-Chuzaichi-cho, Sakyo-ku, Kyoto 606-0021, Japan
⁶ Stazione Astronomica di Sozzago, Cascina Guascona, 28060 Sozzago (Novara), Italy

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 27 April 2026
Accepted: 18 June 2026

Abstract

Context. Interstellar objects are the relics of planetary formation of extrasolar systems. They represent a unique opportunity to study insights of planetary formations around other stars. Since the discovery of 1I/‘Oumuamua in 2017, only two more interstellar objects have been found. 3I/ATLAS is the third, and its cometary nature was confirmed by early observations.

Aims. Long-slit spectra of comet 3I/ATLAS were obtained with the 1.22 m telescope at the Asiago Astrophysical Observatory. The 7’ slit provided sufficient spatial coverage through the coma to investigate the release of atomic and molecular species. We provide the first extensive spatial-distribution study of species in the coma of the interstellar comet 3I/ATLAS, with a total coverage of up to 2.8·10⁵ km. Such spatial information has not previously been reported in the literature. We analysed the Fe I, Ni I, and CN emission features observed at 2.17 AU.

Methods. We extracted the two-dimensional long-slit spectrum and performed the wavelength and flux calibration. From the resulting spectrum, we analysed the cometary emission line spatial distribution and the production rates of the parent molecules through a Haser modelling approach.

Results. Several neutral iron and nickel lines were detected together with the CN B-X (0–0) band. Production rates of CN and C₃ and 3σ NH upper limits were computed as well. We obtained the scale length of nickel, l_d(Ni) = 8.26_−1.48^+2.12 10⁵ km, and iron, l_d(Fe) = 2.48_−0.42^+0.57 10⁵ km. These results imply a slower outflow velocity with respect to Solar System comet values. The relative abundance was found: Ni/Fe = 0.20 ± 0.11. For CN, the obtained scale length is in agreement with typical values adopted for Solar System comets, but the CN parent scale length of l_p(CN) = 4.52_−0.03^+0.03 10⁴ km we found is notably shorter than the usual values assumed or fitted for the CN parent in the literature.

Conclusions. The iron and nickel scale lengths we computed suggest that they originate from a short-lived parent, with no indication of an extended source in the coma. The lifetime of atoms are compatible with photoionisation under active Sun conditions. Our analysis of the CN spatial profile provides the first evidence that CN production in an interstellar comet may not follow the standard behaviour observed in Solar System comets. Our results indicate that CN in 3I likely originates from a combination of known parent molecules together with an additional short-lived component or, alternatively, from a parent species with an outflow velocity significantly lower than typically inferred for Solar System comets.